Flashcards · 7408 · Topic 10

Medical physics flashcards

36 cards on Topic 10, subtopics 3.10.1 to 3.10.6. Pick a subtopic or study them all.

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3.10.1 Physics of the eye

Where is light refracted in the eye, and what is the image on the retina like?: Mostly at the cornea; the lens fine-tunes the focus. The image is real and inverted.
Cones: Work in bright light; give colour vision (three types, peaks near 420, 530 and 560 nm); packed in the fovea; one nerve connection each, so high resolution.
Rods: Work in dim light; one type (peak near 498 nm), so no colour; many share one nerve fibre, so low resolution but high sensitivity.
Myopia vs hypermetropia: the defect and the correcting lens: Myopia: far point too close; corrected with a diverging lens. Hypermetropia: near point too far; corrected with a converging lens.
A myopic eye has a far point of 2.0 m. What is the power of the correcting lens?: u = ∞, v = −2.0 m (virtual image). P = 1/u + 1/v = 0 − 0.5 = −0.5 D (diverging).
Astigmatism: The eye has different power in different planes. Corrected with a cylindrical lens. A prescription gives sphere power, cylinder power and axis angle in degrees.

3.10.2 Physics of the ear

What does the middle ear do?: The ossicles act as a lever, and the oval window has a much smaller area than the eardrum, so the pressure rises.
Inner ear: how is sound turned into nerve impulses?: Vibrations pass into the fluid of the cochlea; hair cells on the basilar membrane produce nerve impulses. The position along the membrane depends on the frequency.
Intensity level equation, and the effect of ×10 and ×2 in intensity: Intensity level = 10 log(I/I₀) dB, with I₀ = 1.0 × 10⁻¹² W m⁻². ×10 in I: +10 dB. ×2 in I: +3 dB.
dB vs dBA: dBA is weighted for the ear's frequency response; dB is not.
Equal loudness curve: Joins sounds that seem equally loud; built by matching tones to a 1 kHz reference. The lowest curve is the threshold of hearing; the ear is most sensitive in the middle of its range.
Effect of noise damage and of age on hearing: Noise: the threshold rises over a band of frequencies (a notch). Age: the threshold rises most at high frequency, and the upper limit falls.

3.10.3 Biological measurement

How is an ECG obtained?: Electrodes on the limbs and chest; skin cleaned and a conducting gel used for low-resistance contact. A high-gain amplifier with high input impedance amplifies the signal, about 1 mV, for display.
ECG waveform: what do the P wave, QRS complex and T wave show?: P: atrial depolarisation. QRS: ventricular depolarisation (the largest). T: ventricular repolarisation.
Why is the QRS complex the largest part of the ECG?: It comes from the ventricles depolarising and contracting, and the ventricles have the most muscle.

3.10.4 Non-ionising imaging

Acoustic impedance and the fraction of ultrasound reflected at a boundary: Z = ρc. I_r/I_i = ((Z₂ − Z₁)/(Z₂ + Z₁))².
Why is coupling gel used in ultrasound scanning?: Air and skin have very different acoustic impedances, so almost all the ultrasound would be reflected at an air–skin boundary. Gel removes the air.
How does a piezoelectric transducer send and receive ultrasound?: A pd pulse makes the crystal vibrate, sending an ultrasound pulse. Returning echoes deform the crystal, producing a pd. Backing material damps the crystal to give short pulses.
A-scan vs B-scan: A-scan: amplitude against time in one direction, giving depths. B-scan: swept probe; brightness shows echo strength, giving a 2D image.
Endoscope: coherent vs non-coherent fibre bundle: Non-coherent bundle: carries light in (illumination). Coherent bundle (fibre positions match at both ends): carries the image out.
MR scanner: the sequence: A superconducting magnet aligns protons, which precess about the field. Gradient coils select the cross-section. RF pulses excite protons; as they de-excite they emit RF signals, which a computer processes into an image.

3.10.5 X-ray imaging

Continuous vs characteristic X-ray spectrum: Continuous: electrons decelerate in the target. Characteristic lines: inner-shell vacancies are filled. The cut-off is at E = eV.
Maximum photon energy and minimum wavelength from a 100 kV X-ray tube: E = eV = 100 keV. λ_min = hc/eV = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (1.60 × 10⁻¹⁹ × 1.0 × 10⁵) = 1.24 × 10⁻¹¹ m.
Effect of tube current, tube voltage and an aluminium filter on an X-ray beam: Current controls intensity. Voltage controls photon energy. The aluminium filter removes soft (low-energy) X-rays, lowering the patient's dose.
Attenuation equation and half-value thickness: I = I₀e^(−μx), with μ the linear attenuation coefficient (m⁻¹). x½ = ln 2/μ. Mass attenuation coefficient μm = μ/ρ.
Flat panel detector: A scintillator converts X-rays to light, photodiode pixels detect it, and the pixels are scanned electronically. Instant, digital, and a lower dose than film.
Intensifying screen vs image intensifier: Intensifying screen: light from a phosphor exposes the film, so less dose is needed. Image intensifier: X-rays → light → electrons → accelerated onto an output phosphor, giving a bright live image.
CT scanner: A rotating X-ray tube with a narrow monochromatic beam and a detector array; a computer builds the image. High resolution and 3D, but costly and a high dose.

3.10.6 Radionuclide imaging and therapy

Properties of a good medical tracer: γ only; a half-life suited to the scan; γ energy that escapes the body yet is stopped in the camera crystal; can be labelled to an organ-seeking compound.
Why is technetium-99m so widely used?: A pure γ emitter (about 140 keV), half-life about 6 h, and easily labelled to many compounds.
Molybdenum-technetium generator: Longer-lived Mo-99 (half-life about 66 h) decays to Tc-99m, which is eluted with saline on site.
Effective half-life: T_P = 6 h and T_B = 24 h. Find T_E.: 1/T_E = 1/T_B + 1/T_P = 1/24 + 1/6 = 5/24, so T_E = 4.8 h, shorter than both.
PET scanning: A positron emitter is used. Annihilation gives two γ photons travelling back to back; coincidence detection locates the line on which the source lies.
Gamma camera: the parts in order: Collimator → scintillator crystal → photomultiplier tubes → position logic → computer.
Photomultiplier tube: Photocathode (photoelectric effect) → dynodes at rising potentials multiply the electrons → a pulse at the anode.
High-energy X-ray therapy: how is healthy tissue protected?: Several beams crossing at the tumour, or rotation about it; beam shaping; shielding; dose split into several sessions.